The invention relates to the field of measurement of liquid levels in groundwater wells, storage tanks, vessels and other liquid containers.
This invention relates to devices known as liquid level detectors, interface probes, and pressure transducers which are used to gauge the depth of groundwater and/or any light non-aqueous phase liquids (LNAPL) or dense non-aqueous phase liquids (DNAPL) i.e. petroleum or solvents respectively, that may be present in groundwater wells, storage tanks, vessels or other liquid containers. Both LNAPL and DNAPL constitute non-electrically conductive fluids.
Traditionally, in accordance with the prior art, monitoring wells are gauged using invasive detection devices with a sensor probe attached to a graduated tape that is wound on a reel. The sensor probe is lowered into the monitoring well casing until the sensor probe comes in contact with the subsurface media i.e. groundwater, in the well. To detect liquids, these devices use an infra-red emitter and detector. When the probe enters a liquid the infra-red beam is refracted away from the detector, which activates an audible alarm and light.
If the liquid is non-conductive (NAPL) the alarm and light are steady. If the liquid is water, a conductive liquid, the water completes a conductivity circuit. This overrides the infrared circuit, and the tone and light are intermittent. The total depth of the well is determined by lowering the sensor probe to the bottom of the well. The measurement is read from the graduated tape and the depth of each media is manually recorded. Errors can be introduced through transcription and by misreading the graduated tape.
If a non-aqueous NAPL such as petroleum is present, the sensor probe and measurement tape can become smeared with petroleum, which in some cases can be difficult to completely remove from the sensor probe and tape. The decontamination process must be thorough to avoid leaving behind any petroleum residues. In accordance with state and federal environmental regulations, these devices must be thoroughly decontaminated between use in each monitoring well to prevent cross-contamination from well to well in subsurface groundwater. This is an important step in the prior process which is time consuming and consequently costly. In addition, when the probe is removed the field personnel gauging the well, run the risk of being exposed to contaminants in the well.
The present invention avoids prior art tape-reading errors and transcription errors by displaying the gauged values on a hand-held digital display unit where they can be stored electronically and later transferred directly to a computer. There is no decontamination procedure necessary, since the sensor tape is part of the well and there is no contact with contaminated groundwater which can be transferred from one well to the next or from the well to field personnel.
Regarding groundwater sampling and well volumes, in order to determine groundwater quality it is necessary to collect samples of groundwater and submit them for laboratory analysis. Prior to collecting a groundwater sample a specified volume of groundwater (typically 3 well volumes) must be withdrawn from the well in order to obtain a sample representative of the aquifer. The volume of groundwater within the monitoring well casing is calculated using the measured depth to water and total depth of the well. In addition it is necessary to determine the volumes of any NAPL that may be present in a well to determine the extent of release of such liquids. The invention disclosed herein, calculates and displays the thickness and volume of water and any LNAPL or DNAPL on the hand held digital display unit.
Aquifer tests such as pumping tests and slug tests are used to determine site-specific aquifer parameters such as hydraulic conductivity, transmissivity and storage coefficients. These tests are performed while groundwater levels in monitoring wells are continuously monitored and recorded. This is typically accomplished through the use of a pressure transducer probe installed in a monitoring well, penetrating the groundwater and attached via chords to a data logger at the surface. These data loggers collect and store the groundwater level data for subsequent determination of aquifer parameters. The pressure transducers must also be decontaminated between each use to prevent cross-contamination.
The invention disclosed herein can also monitor and store groundwater level data during these aquifer tests for subsequent determination of aquifer parameters.
Regarding well locating, it is sometimes difficult to determine if the well being gauged or sampled is the well of interest. This is particularly a problem at sites with numerous wells located relatively close together and installed at similar depths. Groundwater data collected is invaluable in determining site conditions accurately and gauging and sampling the correct well is paramount.
In the aforesaid prior art apparatus, an elongated resistive sensor tape was employed to detect the level of a liquid using the hydrostatic pressure of the liquid in which it is immersed. The hydrostatic sensor consists of a conductive base strip that is partially insulated from a resistive wire that is wound around the base strip to form a helix. The sensor tape is encapsulated in an outer protective envelope to insulate the sensor from the liquid in which it is immersed. When the sensor is immersed in a liquid the hydrostatic pressure of the liquid compresses the envelope. This causes the wire to contact the base strip, which results in a change in resistance of the wire. The resistance of the wound wire corresponds to the distance from the top of the sensor tape to the liquid surface. See U.S. Pat. No. 4,816,799 to Ehrenfried et al., assigned to Metritape Inc. of Littleton. Mass.
A disadvantage of this prior art sensor is that it only can detect the level of a single liquid, e.g. water or petroleum. The sensor can not distinguish between the two in the same well, tank or vessel. In addition, the resolution of the sensor is limited by the minimum spacing between windings of the resistive wire (two hundredths of a foot).
The invention disclosed herein, utilizes an elongated resistive sensor tape that incorporates both a hydrostatic and conductive resistive circuit capable of distinguishing between a conductive liquid (water) and a non-conductive liquid (petroleum) at a resolution of one hundredth of a foot and greater. The resolution is limited only by the smallest spacing possible between contacts of individual thin film transistor circuits in the neighborhood of micrometers.
An elongate thin film sensor tape circuit 1, in FIG. 1, is incorporated into the inside annulus of a groundwater well casing 3 so that is in contact with any liquid present in the well. The sensor tape""s outer jacket has a smooth nonstick surface to prevent build up of highly viscous liquids and includes a conductive resistive circuit that detects the level of water that conducts current in contrast with contaminants. An array of tiny stainless steel electrodes 5 exposed at the surface of the sensor tape at 0.01-foot intervals along the length of the sensor tape, sense the presence of water such that an electrical resistance measured at the top of the well casing is proportional to the depth of the water in the well. The thin film sensor tape circuit also includes a hydrostatic sensing circuit 7 that is sensitive to the actuation pressure of any liquid, (water or NAPL), in which it is immersed. This circuit consists of a network of resistors 9 with an intervening pair of contacts 11 at 0.01-foot intervals. As the level of the liquid rises, the increased hydrostatic pressure of the liquid compresses a movable overlying metal base strip incorporated in the outer jacket of the sensor tape against the contacts such that the electrical resistance measured at the top of the well casing is proportional to the depth of the liquid in the well. Regarding this device, note the hydrostatic sensor of the aforesaid Ehrenfiied patent discussed above. In addition the sensor tape includes a series network of resistors 13 proportional to the total length of the sensor tape sections.
The sensor tape 1 extends along the full length of each well section. The well casing consists of one, two, five or ten-foot modular sections of slotted well screen or blank casing, which have the sensor tape incorporated therein. The modular sections 3 can be connected using various lengths to obtain the desired well depth. Specialized bottom and top cap sections, 15 and 17 respectively, are connected to complete the sensor circuit. The top cap, in addition to completing the circuit, houses the display connector and a nonvolatile memory circuit that stores pertinent well information such as: the identification number; installer; installation date; depth; diameter; and top of casing elevation. Electrical interconnects are made in the coupling of the modular well casing sections using a specialized connector and double o-ring arrangement to prevent exposure of the connections to the liquid in which it is immersed. A hand held digital processor unit measures the electrical resistances at the surface of the well casing, calculates and displays the level, thickness and volume of water and any light LNAPL or dense DNAPL.